In recent years, the Thin Film Transistor-LCD (TFT-LCD) has been rapidly developed and applied widely. For the TFT-LCD in the mainstream market, three types, which respectively are Twisted Nematic (TN), Super Twisted Nematic (STN), In-Plane Switching (IPS) and Vertical Alignment (VA) can be illustrated. The VA liquid crystal display possesses extremely high contrast than the liquid crystal displays of other types, which can reach up to 4000-8000 in general. It has very wide application in large scale display, such as television or etc.
The reason why the VA liquid crystal display possesses extremely high contrast is that the liquid crystal molecules are vertically aligned to the substrate surface, and no phase difference exists, and light leakage is very small, and the dark state brightness is extremely small at the dark state without applying electricity. The lower the brightness at the dark state can be, the higher the contrast is according to the contrast calculation formula. For vertically aligning the liquid crystal molecules of the VA liquid crystal display to the substrate surface, it is demanded to implement vertical alignment treatment to the liquid crystal molecules. The most common way is to coat vertical alignment solution on specific areas of surfaces of the upper, lower substrates, and the alignment solution generally comprises a large amount of chemical solution NMP (N-methylpyrrolidone) and Polyimide (PI), and then to bake the substrates for a long period at high temperature (generally above 200 degrees) for curing the solvent in the alignment solution. Thus, PI alignment layers are formed on the surfaces of the substrates. As shown in FIG. 1, the traditional VA liquid crystal display comprises an upper glass substrate 100, a lower glass substrate 200 oppositely located to the upper glass substrate 100, a liquid crystal layer 400 sandwiched between the upper glass substrate 100 and the lower glass substrate 200, PI alignment layers 300 formed on a surface of the upper glass substrate 100 facing to the glass substrate 200 and a surface of the lower glass substrate 200 facing to the upper glass substrate 100. However, because the VA liquid crystal display utilizes vertical twist liquid crystals and the birefraction difference of the liquid crystal molecules is larger, the issue of the color shift under large view angle is more serious.
For earning better wide view angle property for the VA liquid crystal display panel to improve the color shift issue, the multi-domain VA (MVA) technology is commonly utilized, which is to divide a sub pixel into many districts and drive the liquid crystals in respective districts to lie down toward different directions as applying voltage. Thus, the watch results from respective directions can be equal. There are many methods for realizing the MVA technology. Please refer to FIG. 2, FIG. 3 and FIG. 4. One of the methods is to process onside of the ITO pixel electrode 520 to be a pozidriv pattern. The common electrode 510 is a plane electrode which has uniform thickness and is uninterruptedly continuous. With the special ITO pixel electrode pattern, the tilt electric field can induce the liquid crystal molecules 400 to fell down toward different directions.
FIG. 2 is a top view diagram of one side of a lower substrate 20 in an MVA type liquid crystal display panel. 610 and 620 respectively are a scan line and a data line. The ITO pixel electrode 520 comprises a (cross) keel 511 as being a main piece and a pattern of pixel electrode branches 512 respectively extending in directions of 45°, 135°, −45° and −135° from the (cross) keel 511 relative to the horizontal direction with spaced slits. FIG. 3 is a sectional diagram of an MVA type liquid crystal display panel corresponding to A-A portion shown in FIG. 2. The pixel electrode 520 with slits is positioned on the flat lower passivation layer 600. The plane common electrode 510 is positioned on the flat upper passivation layer 600. The PI alignment layers 300 cover on the pixel electrode 520 and the common electrode 510.
According to the transmittance formula of the VA liquid crystal display panel:
                              T          =                                    1              2                        ⁢                          sin              2                        ⁢            2            ⁢                          ΔΦsin              2                        ⁢                          Γ              2                                      ,                            (        1        )            wherein T is the penetration rate, and Δφ is the included angle between the long axis of the liquid crystal and the polarizer, of which the efficiency is the maximum as the angle is 45°; Γ is the phase difference, i.e. the modulation result to the polarized light with the liquid crystal molecules driven by the electrical field.
The calculation formula of Γ is:Γ=cos(a)*2π*Δn*d/λ  (2)wherein a is the included angle between the long axis of the liquid crystal and the normal line of the substrate, and the volume is determined according to the electrical fielding affecting the liquid crystal molecules, and d is the cell gap, and Δn is the refractivity difference of the long, short axes of the liquid crystal.
According to the penetration rate formula, in the four areas of the sub pixel, the pixel electrode 520 comprises a pattern of pixel electrode branches 512 respectively extending in directions of 45°, 135°, −45° and −135° relative to the horizontal direction with spaced slits (the direction of the upper, lower polarizers respectively are 0°, 90°. The long axes of the liquid crystal molecules will respectively fell down toward the directions of 45°, 135°, −45° and −135° relative to the horizontal direction. In the penetration rate formula sin22ΔΦ=1, the maximization of the penetration rate can be achieved.
However, the liquid crystal molecules 400 in the area corresponding to the (cross) keel 511 of the pixel electrode 520 as shown in FIG. 2 always cannot fell down as that the liquid crystal molecules in the areas corresponding to the pattern of the pixel electrode branches 512 with spaced slits fell down toward the directions of 45°, 135°, −45° and −135° relative to the horizontal direction. Thus, as shown in FIG. 4, the liquid crystal molecules 400 in the area corresponding to the (cross) keel 511 tilt toward 0° or fell down toward 90° to make sin22ΔΦ=0 in the penetration rate formula. The display is in an opaque state to cause the entire penetration rate of the liquid crystal display panel to descend.